The mammalian urinary tract is composed of the kidneys, which filter blood and remove excess fluids, solutes and metabolic by-products from systemic circulation and a conduit system, that carries these wastes out of the body. The bladder and urethra form independently of the kidney and ureter, connecting at midgestation to form a patent outflow tract. Ureter insertion, the connection process, involves a complex series of events in which the ureter orifice moves from its original insertion site in the common nephric duct (CND), the most caudal Wolffian duct segment, to the urogenital sinus. Defects in this process are associated with a wide spectrum of birth defects in humans affecting 1-2% of the population, which, include vesicoureteral reflux and obstruction that can damage the kidneys and cause end stage renal disease. We find that insertion of the ureter into the bladder depends on epithelial remodeling of the CND that enables the ureter to align with and fuse with the bladder epithelium, its final insertion site. Once this occurs, apoptosis generates a patent urinary outflow tract. Using mouse models, we have shown that ureter insertion depends on retinoic acid and that retinoids act by inducing transcription of Ret, a gene required for kidney development and for ureter insertion. In the current proposal, we will determine in Aim 1, whether vitamin A controls Ret expression directly, inducing its transcription in CND cells, or indirectly, by inducing urogenital sinus signals that control Ret expression in the CND. In Aim 2 we will characterize the cellular events mediated by Ret by analyzing ureter insertion in Retk- mutants and, by generating chimeric embryos from Ret mutant and wild type cells to determine whether there is a cell-autonomous requirement for Ret in CND cells. Finally, we will examine the requirement for apoptosis during ureter insertion by testing whether Bcl2, a potent anti- apoptotic molecule, can disrupt urinary tract formation and by analyzing mutants lacking caspase-9 and -8 crucial mediators of the intrinsic and extrinsic apoptotic pathways, respectively, the major apoptotic signaling cascades. PUBLIC HEALTH RELEVANCE Urinary tract abnormalities are relatively common health problems in humans, affecting 1- 2% of the general population. These malformations can result in back-flow of urine to the kidney that can cause severe damage and lead to end stage renal disease. We will study cellular and genetic mechanisms of normal urinary tract formation and identify defects that lead to urinary tract abnormalities including reflux and obstruction using mouse models. These findings should help in our general understanding of the abnormalities underlying urinary tract birth defects in humans and may help to identify new ways to intervene or repair urinary tract malformations in newborns.